1. Field of the Invention
The present invention relates to wavelength-division multiplexing in passive optical networks.
2. Description of the Prior Art
FIG. 1 shows a block diagram of parts of a conventional communications network employing a passive optical network (PON). The communications network 1 has an optical line termination unit (OLT) 2 and a plurality of optical network units (ONUs) 41 to 44. The ONUs 41 to 44 are connected to the OLT 2 by a passive optical network 6 which consists of optical fibre links 8 and optical splitters 10. The OLT 2 is located at the so-called xe2x80x9chead endxe2x80x9d of the PON 6 and serves to connect the PON to a core network. Customers or subscribers are connected to the ONUs.
The communications network 1 shown in FIG. 1 may be employed as part of an asynchronous transfer mode (ATM) communications network. In this case, the so-called xe2x80x9cdownstream trafficxe2x80x9d, i.e. the data (ATM cells) to be transmitted from the OLT 2 to the ONUs 41 to 44, is broadcast at a single optical wavelength xcex1 to all of the ONUs and each ONU then selects the appropriate ATM cells destined for it and ignores any other cells.
In the upstream direction, from the ONUs to the OLT, the individual signals from the ONUs 4 are interleaved in a predetermined time-division multiple-access (TDMA) format. For example, in the TDMA format shown in FIG. 1 itself, each ONU 4i is allocated its own time slot TSi within a frame FRUP. All upstream traffic is at a single wavelength xcexx which may be the same as the downstream wavelength xcex1 or may be different from xcex1. The upstream traffic from the ONUs to the OLT will generally be of a much lower data rate than that of the downstream traffic. The maximum capacity of the PON 6 is therefore required to correspond to the maximum data rate of the downstream traffic.
The PON 6 may be of the two-fibre type which is effectively two passive optical networks (two sets of fibre links 8 and optical splitters 10) used in parallel, one for the downstream traffic and the other for the upstream traffic. The capacity of the upstream-traffic PON can, if desired, be lower than that of the downstream-traffic PON.
Alternatively, the PON 6 shown in FIG. 1 may be of the single-fibre type which uses just one set of fibre links 8 and optical splitters 10 to connect the OLT to the ONUs; in this case a return path from the ONUs to the OLT is provided by time-division multiplexing the downstream and upstream traffic over the single-fibre PON. Again, depending on the time-division format used, the effective capacity available to the upstream traffic may be made lower than the effective capacity available to the downstream traffic.
For simplicity, the embodiments described specifically in the present application will make use of the two-fibre type PON but as will be readily apparent the present invention can also be used with single-fibre type PONs.
In order to increase the maximum capacity of a passive optical network it is possible to employ wavelength-division multiplexing. If, for example, the downstream traffic capacity of the PON 6 is fmax when the downstream traffic is broadcast on a single wavelength, the capacity of the PON 6 is increased to N x fmax when N optical signals at different respective wavelengths are employed to broadcast the downstream traffic.
Using this technique it would be possible to pre-assign each ONU with its own unique wavelength on which to receive data from the OLT 2. However, such an approach is unsatisfactory for two reasons. Firstly, even with state-of-the-art technology a maximum of 32 different wavelengths is presently possible, whereas it may be desired to support over 100 ONUs from the same OLT. Secondly, the downstream traffic requirements for the different ONUs are not fixed over time, so that at any given time the amount of downstream traffic can vary greatly from one ONU to the next. At certain times, some of the ONUs may have no downstream traffic at all. Preassigning all ONUs with an equal or fixed amount of capacity is therefore potentially wasteful of the overall downstream traffic capacity of the PON.
According to a first aspect of the present invention there is provided a communications network including: an optical transmitter for generating a plurality of optical signals having different respective wavelengths, each said optical signal carrying data, and wavelength-division-multiplexing the optical signals; and a plurality of optical receivers connected to the optical transmitter by way of a passive optical network for receiving the wavelength-division-multiplexed optical signals, each receiver having wavelength selection means operable in dependence upon control information sent from the transmitter to the receiver concerned (for example by way of the passive optical network) to select one of the optical signals of the said plurality, and also having detection means for processing the selected optical signal to derive therefrom the data carried thereby.
In such a network the downstream capacity of the passive optical network can be shared flexibly by the different optical receivers.
For example, in the optical transmitter the data to be transmitted to the optical receivers may be allocated to the optical signals of the said plurality dynamically in dependence upon the respective amounts of data which it is desired to transmit to the different optical receivers in a particular time frame. In the optical transmitter, data destined for the optical receivers may, for example, be buffered in queues corresponding respectively to the different optical receivers and the amounts of data for the different optical receivers can then be determined from the queue fill levels.
The control information is preferably sent from the optical transmitter to the optical receiver concerned by way of the passive optical network but may alternatively be sent by way of further communications paths linking the transmitter to the receivers. In this case, the control information could be embodied in radio signals, or in electrical signals carried by dedicated landlines.
To reduce the amount of control information required to be transmitted, the control information preferably specifies only changes in optical signal selection to be made by the optical receivers.
The control information may be carried as overhead information by the optical signals. This keeps the cost of the optical receivers down because the control information can be received through the selected optical signal and detected using the same detector that detects the data.
In one embodiment, the control information relevant to a given optical receiver is carried as overhead information by all of the optical signals. This keeps the design of the optical transmitter simple because it does not need to keep track of the optical signal that the given optical receiver has currently selected. However, the broadcast of the control information on all optical signals is wasteful of the downstream capacity and accordingly in another embodiment the control information relevant to a given optical receiver is carried as overhead information only by the optical signal currently selected by the said given optical receiver.
In this case the optical transmitter preferably has selection storing means for storing the respective current optical-signal selections made by the optical receivers, and overhead information adding means operable, when control information is to be transmitted to one of the said optical receivers, to determine from the stored current optical-signal selections the optical signal of the said plurality that is currently selected by that optical receiver, and to cause the control information to be carried as overhead information only by the determined optical signal.
In another technique for increasing the throughput of data, two or more of the said optical signals may be used to carry simultaneously, as overhead information, different control information relevant to different respective said optical receivers.
It is also possible for the control information to be transmitted from the optical transmitter to the said optical receivers by a further optical signal, having a wavelength different from that of each of said optical signals of the said plurality, that is wavelength-division-multiplexed with the optical signals of the said plurality. This avoids the reduction in data throughput that arises from the use of overhead information in the data-carrying optical signals to transmit the control information.
The control information may be divided into fields corresponding respectively to the said optical signals, each field specifying at least one optical receiver that is to select the corresponding optical signal. The fields then implicitly identify the optical signal to be selected by an optical receiver. The fields are preferably ordered differently in the overhead information carried by the different optical signals such that, for each optical signal, the last field in the overhead information is the field that corresponds to the optical signal concerned. This means that the fields relating to optical signals other than the currently-selected optical signal arrive at the optical receiver before the field relating to the currently-selected optical signal. This can be effective in allowing more time for the optical receivers to select a new optical signal.
To simplify the transmission of data, the data is preferably transmitted in predetermined time slots from the optical transmitter to the optical receivers, and in each time slot respective units of data are transferred substantially synchronously via the optical signals from the optical transmitter to the optical receivers. In this case, the overhead information fields may contain respectively the control information for the different data units that are to be transmitted in the same time slot by the different optical signals.
Each data unit may comprise, for example, at least the payload portion of an ATM cell. In this case, the control information preferably includes addressing information from the ATM cell headers. In an ATM system, such addressing information already implicitly identifies the optical receiver to which the cell payload concerned is to be sent so that there is no need to generate additional, special information for designating the optical receivers.
The overhead information may be transmitted in the intervals between successive time slots. This provides a built-in guard band, between the end of one time slot and the start of the next time slot, in which a new optical signal can be selected. Alternatively, or in addition, the data units transmitted in successive time slots (e.g. four time slots) by each optical signal are combined with the overhead information to form a frame. Such a frame structure can reduce the ratio of overhead information transmission time to data transmission time.
Preferably, the control information is sent in advance of the time slot to which it relates, so as to provide extra time for an optical receiver to effect selection of a new optical signal. Sending the control information in advance is also beneficial if an optical receiver is about to enter a xe2x80x9cquiet periodxe2x80x9d in which it does not require any share of the available bandwidth. In this case, if the optical receiver concerned is required to select a new wavelength to receive data in the first active time slot after the quiet period is over, then the necessary control information specifying the new selection can be sent before the commencement of the quiet period. The control information may, for example, be provided at the head of a frame and specify in advance the optical signal selections (or just the changes in selection) to be made in the time slots of that frame or even in the time slots of a subsequent frame. The control information may also specify the time slots in which the optical receivers should change their optical signal selections. Alternatively, the control information may always be provided one time slot (or a predetermined number of time slots) ahead of a required selection or change in selection. In all cases, each optical receiver may be provided with buffering means for holding the received control information until the time slot concerned.
According to a second aspect of the present invention there is provided an optical transmitter, for connection by way of a passive optical network to a plurality of optical receivers, including: signal transmission means for generating a plurality of optical signals having different respective wavelengths, each said optical signal carrying data, and wavelength-division-multiplexing the optical signals; and control information generation means for generating control information to be sent (for example by way of the said passive optical network) to the said optical receivers to designate which of the optical signals of the said plurality each receiver is to select to derive therefrom the data carried thereby.
The signal transmission means may include: a plurality of transmitter devices corresponding respectively to the said optical signals, each transmitter device being connected to the said transmission control means for receiving therefrom the data allocated by the transmission control means to its corresponding optical signal and being operable to modulate its corresponding optical signal with the allocated data; and wavelength-division-multiplexing combiner means coupled to each of the said transmitter devices for wavelength-division-multiplexing the said optical signals.
If desired, a further transmitter device may be coupled to the said control information generation means and also coupled to the said wavelength-division-multiplexing combiner means for generating a further optical signal, having a wavelength different from that of each of the optical signals of the said plurality, which further optical signal carries the said control information.
According to a third aspect of the present invention there is provided an optical receiver, for connection by way of a passive optical network to an optical transmitter which wavelength-division-multiplexes a plurality of optical signals having different respective wavelengths when in use, each said optical signal carrying data, the optical receiver including: wavelength selection means operable in dependence upon control information sent from the said optical transmitter to the optical receiver (for example by way of the said passive optical network) to select one of the optical signals of the said plurality; and detection means for processing the selected optical signal to derive therefrom the data carried thereby.
The wavelength selection means preferably include a tunable filter connected for receiving the wavelength-division-multiplexed optical signals and operable, in dependence upon a control signal derived from the received control information, to deliver to the said detection means only the said selected optical signal. In this case, only a single detector is required, making the design of the optical receiver simple and cost-effective. If desired, however, the optical receiver may be designed to select two or more optical signals simultaneously (for example by providing two or more tunable filters and corresponding detectors) so as to increase the maximum share of the downstream capacity that can be allocated to the optical receiver.
According to a fourth aspect of the present invention there is provided a communications method, for use in a communications network including an optical transmitter which is connected to a plurality of optical receivers by way of a passive optical network, including: at the optical transmitter, generating a plurality of optical signals having different respective wavelengths, each said optical signal carrying data, and wavelength-division-multiplexing the optical signals; and at each optical receiver, receiving the wavelength-division-multiplexed optical signals and selecting one of them in dependence upon control information sent from the optical transmitter to the optical receiver concerned (for example by way of the passive optical network), and processing the selected optical signal to derive therefrom the data carried thereby.